The present invention relates to a method of detecting and controlling detonation of an internal combustion engine.
As is known, in certain operating conditions, internal combustion engines are subject to high-intensity detonation, i.e. spontaneous combustion of the mixture, which occurs in many engine cycles, and results in impaired efficiency, dangerous overheating and reduced life of the engine, as well as possibly in sudden failure of certain engine components.
Various systems are therefore being studied, or have already been proposed, to detect detonation and act accordingly on combustion parameters to reduce its effects and the likelihood of it occurring.
Such systems are mainly based on directly or indirectly monitoring the pressure cycle pattern in the cylinder as a function of the position of the piston inside the cylinder. The pressure cycle pattern is bell-shaped, reaching a peak close to the top dead-center position, which peak is a classic rounded shape when combustion is normal (FIG. 1a), and has numerous indentations in the presence of detonation (FIG. 1b).
Information by which to detect detonation is obtained by analyzing the indentations.
Some known methods employ sensors located inside the combustion chamber to directly determine the amplitude of the indentations. While providing for highly accurate, reliable amplitude values, such a sensor arrangement calls for sophisticated high-cost technology and is therefore only suitable for laboratory or prototype application.
Other methods employ sensors located, and for detecting vibration, on the crankcase. Though technically simpler and cheaper, the values obtained by such sensors are subject to greater interference than those detected directly, on account of the vibration measured on the crankcase also being the result of other phenomena besides crankcase-filtered variations in pressure inside the cylinder.
Patent GB-A-2 265 006 filed by Nippondenso Co Ltd, for example, relates to a detonation control system featuring detonation sensors on the crankcase, and which detects detonation by comparing the intensity of the sensor signal with a decision threshold, and provides for eliminating detonation by accordingly controlling engine operating parameters, such as injection advance. More specifically, the system performs a logarithmic conversion of the intensity of the sensor signal, determines its distribution, calculates a value corresponding to the standard deviation of the distribution, and compares the value with a threshold calculated on the basis of the previously calculated value and a mean value of said distribution.
The distribution is determined by processing numeric values representing the amplitudes of spectral components obtained by narrow-band filtering the output signal of the sensors. The output signal, in fact, comprises numerous harmonics, and narrow-band filtration is employed to only extract the highest-energy harmonic.
It is important to note that, in the above patent, the detonation threshold is adapted at each cycle.
In some cases, however, controlling operation of the engine on the basis of one frequency in the spectrum and one engine cycle at a time may prove restrictive.
That is, considering only one frequency may result in others, different from the one considered but generated all the same by detonation, being overlooked, while control designed solely to eliminate local detonation cycle by cycle may not always be the best solution, in that, in terms of engine operation and efficiency, it sometimes pays to tolerate a given number of detonations every so many engine cycles and/or in only some of the cylinders. Trace detonation, in fact, by enabling high efficiency of the engine, is a favourable engine operating condition, and therefore one to be encouraged.
Finally, adapting the decision threshold calls for numerous numeric processing operations, which further complicate implementation of the method.
One proposal to eliminate the aforementioned drawbacks is the detonation detecting method described in Patent Application WO-97/21084 filed on Nov. 29, 1996 by the same Applicant and published on Dec. 06, 1997.
The detection method in the above patent application provides for wide-band filtering and rectifying the output signal of an acceleration sensor on the crankcase; integrating the rectified signal in a time window appropriately located with respect to the combustion considered; and effecting a logarithmic conversion of the integration result.
Repeating the above operations for each combustion of the engine provides for defining a distribution of values, of which are calculated, at each combustion, a mean value and a numeric value correlated to the standard deviation on the basis of previous combustion values. On the basis of the numeric value and a predetermined threshold value which is constant alongside variations in engine speed, a detonation coefficient value is then calculated at each engine combustion, and which indicates the detonation xe2x80x9cpropensityxe2x80x9d of the engine.
It is an object of the present invention to provide an improved detonation detecting method as compared with that described in Patent Application WO-97/21084.
According to the present invention, there is provided a method of detecting and controlling detonation of an internal combustion engine, comprising the steps of:
a) acquiring a vibration signal proportional to the intensity of vibration on the crankcase;
b) wide-band filtering said vibration signal to generate a first intermediate signal;
c) rectifying said first intermediate signal to generate a second intermediate signal;
d) integrating said second intermediate signal to generate a first numeric value;
e) calculating a logarithm of said first numeric value to obtain a second numeric value;
f) calculating a mean value as a function of said second numeric value;
characterized by comprising the steps of:
g) calculating a difference between said second numeric value and said mean value to obtain a third numeric value;
h) comparing said third numeric value with a predetermined threshold value;
m) determining the presence of detonation in the event said third numeric value has a first predetermined relationship with said predetermined threshold value;
n) forming a detonation index indicating the behaviour of the engine in terms of detonation; and
o) calculating, from said detonation index, a correction value to be added to the spark lead.